1. Field of the Invention
The invention relates to apparatus for operating high speed electronic devices at cryogenic temperatures, and more particularly, for interfacing such devices electrically to room temperature apparatus.
2. Description of Related Art
U.S. Pat. No. 4,401,900 to Faris, entitled "Ultra High Resolution Josephson Sampling Technique," shows a sampling technique with a time resolution of 5 picoseconds and sensitivity of 10 uV. This was demonstrated experimentally using a cryogenic sampling system. The time resolution of this system is extendable to the sub-picosecond domain, limited ultimately by the intrinsic switching speed of the Josephson device used as the sampling gate. This switching speed can in principle be as little as 0.09 picoseconds. The sampling technique is not restricted to measuring only those waveforms produced in a cryogenic environment. Rather, it can be used to measure waveforms from various sources, such as x-rays, optical photons, or electrical waveforms produced by room-temperature sources, if a suitable interface is available.
In order to measure electrical waveforms produced by room temperature devices, or indeed to interface any low temperature electronic device to a room temperature electronic device, the interface scheme must satisfy the electrical, mechanical, and temperature constraints discussed below:
Electrical Constraints. When operating at high frequencies and extremely short pulse durations, any power lost in the transmission line between the low temperature circuit and the room temperature circuit will degrade the signal transfer. This degradation appears as pulse dispersion or pulse spreading. To minimize loss, the transmission lines should be made of a low resistance material, be as short as possible, and have the largest possible cross sectional area. The latter constraint is limited by the further constraint that the width of the transmission line should not exceed the wavelength of the maximum frequency of interest, because larger conductors will waveguide and cause geometric losses.
Mechanical Constraints. Since one end of the transmission line will be operating at extremely low temperatures and the other end will be operating at room temperatures, it is important that the transmission line be able to withstand that temperature difference. Thus, the bond between the transmission line and the low temperature device should be able to withstand that low temperature, and the seal through which the transmission line passes between the low temperature volume and the room temperature volume should also be able to withstand the necessary temperatures. These elements should also be able to withstand repeated cycling from room temperature to low temperature for maintenance, replenishment of helium supply, and general everyday use. Additionally, the temperature coefficient of expansion of the transmission lines should closely match that of the low temperature device, and the construction should be such as to permit the apparatus to tolerate vibration and temperature-induced changes in transmission line length (collectively referred to herein as "movement").
Temperature Constraints. In order to prevent extensive heat transfer from the room temperature volume to the low temperature volume, the transmission lines should be as long as possible. This is directly contrary to the electrical constraints which favor short transmission lines. The transmission lines should also be made of a material which has low thermal conductivity. Since low thermal conductivity usually implies low electrical conductivity, this constraint, too, is contrary to the electrical constraints.
Workers in the field of superconducting electronics typically achieve the necessary temperatures by immersing their circuits in liquid helium. See, for example, Hamilton, "High-Speed, Low-Crosstalk Chip Holder for Josephson Integrated Circuits," IEEE Trans. on Instrumentation and Measurement, Vol. IM-31, pp. 129-131 (1982). The arrangement shown therein involves attaching several coaxial cables to a Josephson Junction chip which is to be immersed in a liquid helium dewar. See also Hamilton et al., IEEE Transactions on Magnetics, MAG-17, pp. 577-582 (1981), in which a low-temperature chip is inserted partially inside a coaxial line to couple the signals therethrough to the roomtemperature devices. Although not mentioned in the reference, it is believed that the low-temperature chip is then immersed in liquid helium. Both arrangements are constrained to have large coaxial lines which have high thermal conductivity. In order to avoid heat losses, the lines are therefore constrained to be long. In addition, these arrangements cannot be adapted easily to planar chips. Furthermore, at least the latter system is constrained to couple only one line to a chip, which limits the system in utility.
An attempt to deal with the constraints described above appears in U.S. Pat. No. 4,498,046 to Faris. The interface described therein includes a pass-through liquid-helium-tight vacuum seal which consists of a flange and two half-cylindrical fused quartz portions, unequal in length, which act as a pass-through plug from a liquid-helium filled cryostat to a vacuum chamber. Fused quartz, while thermally non-conductive, forms a low loss dielectric substrate for conductive copper striplines which are patterned on the flat surface of the longer portion. The coefficient of expansion of fused quartz is small and relatively well matched to that of silicon, which is used for Josephson and semiconductor chip substrates.
The two fused quartz half-cylinder portions of the pass-through plug are arranged so that the portion with the copper striplines extends sufficiently beyond its mating half-cylinder portion on both ends to provide two platforms at opposite ends of the plug. The low temperature semiconductor chip or device is mounted on one of these platforms and the room temperature chip or device is mounted on the other. The cylindrical geometry was chosen in order to minimize stress on cement used to seal the chamber wall around the pass-through. The planar nature of the striplines allows low inductance connections to be made directly to the two chips which are also planar. The low inductance contacts are copper spheres or other rigid probes, about 100 um in diameter or smaller, which penetrate solder pads on the chips when forced into contact by mechanical pressure. The wall of the cryostat is sealed around the pass-through with a thin layer of non-conductive cement. In operation, the two chips are mounted on the platforms and the pass-through is inserted through the cryostat wall such that the low temperature chip is immersed in liquid helium in the cryostat and the room temperature chip is disposed inside the vacuum chamber. A heating element and thermocouple are placed near the position of the room temperature chip in order to warm it. This chamber must be evacuated in order to prevent frosting of water and other gases on the plug, and also to provide adequate insulation for the cryostat.
The '046 apparatus has numerous problems which render it costly, unreliable and impractical to use in most applications. First, the only method described in the '046 patent for cooling the low temperature device involves immersing it in liquid helium. It is advantageous, however, to be able to cool such devices using a closed cycle refrigerator (CCR), which is a refrigeration device that is complete unto itself, and is simply plugged into an ordinary AC wall socket.
Second, the apparatus requires at least two seals, one between the cryostat and the vacuum chamber, and one between the vacuum chamber and the external environment. At least the first of these seals is extremely difficult to create, because it must operate at cryogenic temperatures, must be able to be cycled many times between cryogenic and room temperatures, and must be able to withstand a certain amount of vibration without breaking. Due to the small size of the helium atom, it can pass through extremely small cracks in the seal and can even pass through most materials which are not cracked. This severely limits the types of seals which can be used.
Third, since the low temperature chip is fabricated on a silicon substrate and the transmission line is fabricated on a fused quartz substrate, the two elements must usually be made separately and then mechanically and electrically bonded together. These additional steps are costly. In addition, even though their respective temperature coefficients of expansion are close, the mere fact that the materials are different requires some mismatch which degrades the electrical connection and the mechanical reliability of the bond.
Fourth, because multiple sealed layers of chambers and insulating material are required, the transmission line which carries electrical signals between the two chips must be very long.
Fifth, the pass-through of the '046 apparatus has to be cylindrical in order to obtain a good seal. This renders it difficult to manufacture, and requires special geometries such as that shown in FIG. 3E of the '046 patent.
Finally, the chips used in the '046 apparatus cannot be easily plugged in or out in order to change them.
It is known in the field of optics that devices which need to operate at extremely low temperatures may be placed in thermal contact with a cold surface which is inside a vacuum chamber. A product which may be used for this application is the Heli-Tran, made by Air Products and Chemicals, Inc., Allentown, PA. It comprises a flexible insulated tube connected at one end to a liquid helium dewar. The free end of the tube is closed and terminates in a metal block to which a sample may be attached. The sample and the metal block are disposed inside a vacuum chamber attached to the end of the tube. Until now, however, such a product has not been used in connection with a low temperature circuit to be connected with a room temperature circuit by a high performance transmission line. See also U.S. Pat. No. 3,894,403 to Longsworth, in which an apparent variation of the Heli-Tran structure is shown cooling a superconducting magnet. Even there the magnet is immersed in liquid helium.
The inability of the above arrangements to effectively satisfy the constraints described above derives in large measure from a belief among workers in the field that immersion in liquid helium is the only feasible way to cool a low temperature circuit. In fact, however, the method and apparatus of the present invention is far more effective and better satisfies the constraints.